A great deal of attention has been directed to the applications of travelling-wave tubes (TWTs) as channel amplifiers and at this time they are currently being used for frequency ranges from about 2 Gigahertz (GHz) to 16 Ghz. One phenomenon observed in connection with TWTs is that they produce higher gains and power output at the center of their operational band. That is, TWTs generally exhibit parabolic gain shapes where the gain is at a maximum at the center operating frequencies.
Many electrical systems, however, require constant gain and power output across their operational band. For this reason, components such as TWTs, which exhibit undesirable amplitude and/or gain variation across a frequency band, are typically "equalized" by an equalizer circuit to reduce the amplitude or gain variation to less than a few dB over relatively broad frequency bands.
Equalizers are either passive or active circuits having a predictable, controlled attenuation slope or characteristic versus frequency. Depending upon the characteristic of the signal they equalize, equalizers fall into one of two categories: parabolic or linear. Many conventional equalizers are of the low voltage standing wave ratio (VSWR) or absorptive type consisting of a fixed parabolic equalizer and an absorptive fine grain equalizer (FGE). Often, the bandwidth range of conventional equalizers is too narrow to correct the gain variations across the frequency bands of certain devices, for example TWTs. In an effort to equalize the gain of such devices, hybrid variable coaxial equalizers have been employed.
Hybrid variable coaxial equalizers generally consist of a main transmission line between an RF input and an RF output, wherein the transmission line has multiple transverse electromagnetic mode (TEM) resonant shunt branches coupled along its length. A fixed equalizer has fixed lengths for each shunt branch or stub tuner with a lossy coating at the end of each stub cavity. By contrast, a variable coaxial equalizer has variable stub lengths. The amount of coupling between the input line and shunt stubs is varied, by mechanically adjusting the position of a lossy coated plug at the end of a stub cavity, so as to alter the equalization profile and produce the desired flattening of the gain. For variable coaxial equalizers, coverage of up to two and one half (2.5) octaves is common with up to 20 watts of power capability. A coaxial equalize while tunable to compensate for a variety of device gain responses, still possesses certain disadvantages. The principal drawbacks are large size and weight, and high cost. Typical dimensions for a variable coaxial equalizer operating at 6-18 Ghz, 5 watts with 15 dB mid band attenuation are on the order of 1.25.times.1.0.times.0.5 inches and weight about 100 grams or more.
In co-pending application Ser. No. 07/942,728 filed Sep. 9, 1992 issued on Feb. 1, 1994 as U.S. Pat. No. 5,283,539 and entitled MONOLITHIC COMPATIBLE, ABSORPTIVE, AMPLITUDE SHAPING NETWORK, there is disclosed a miniaturized variable equalizer circuit which can be fabricated either as a hybrid of microstrip transmission lines and surface mounted resistors, or as part of a completely monolithic device which includes microstrip transmission lines. The equalizer disclosed therein uses a band-stop filter configuration, but replaces the capacitive coupling with resistive coupling in the form of resistive means coupled to one or more stub tuners. The resistive means is utilized to de-Q the stub tuner, thereby permitting the attenuation profile of the equalizer circuit to be selectively determined. Specifically, the amount of resistive coupling controls the depth of the equalization curve and changing the length of the resonators moves the center frequency of the equalizer. While providing a smaller, lighter, and less expensive variable equalizer, the device disclosed in the aforementioned application is unable to shift its loss profile to different frequencies.
It is an object of this invention to provide an equalizer circuit which may be configured as a microwave integrated circuit (MIC) or a monolithic microwave integrated circuit (MMIC) and which achieves broadband performance without any of the aforementioned limitations found in conventional equalizers.